The Universal Serial Bus (USB) standard was developed to provide personal computer users with a single hardware and software interface for connecting a wide range of peripherals to desktop and laptop computers. USB has become the interface of choice for peripheral devices because it offers simple connectivity and a standardized interface that has proven effective for communicating with a vast array of peripheral devices. In addition to providing a standardized interface for communications between a peripheral device and a host device, USB provides the ability to transfer power bi-directionally via a USB connection.
Building on advances in battery technology, peripheral devices powered at least in part by internal, rechargeable batteries have become common in the marketplace. USB has become a popular mechanism for charging these battery-powered peripheral devices. For many peripheral devices, USB is the sole interface provided for charging the peripheral's internal batteries. Some USB peripheral devices have no internal batteries and rely strictly on the power provided via a USB connection in order to operate.
From a hardware perspective, host devices commonly provide a USB interface via a hub that typically has two or more USB ports. In general, a USB hub multiplies the number of USB ports that can be supported by a host device. Each of the USB ports supported by a hub is a “power port” that, in additional to supporting data transfers, also provides the capability of powering and/or charging a connected USB peripheral device. Certain USB ports are configured as dedicated charging ports that do not support data transfers and are strictly used for transferring power, making these ports especially adapted for charging the batteries of a peripheral USB device. Certain hubs include an internal power supply and are thus “self-powered.” Hubs that are self-powered must include over-current protection in order to limit the possibility of allowing potentially harmful current levels to be drawn by peripheral devices connected to the hub.
In devices compliant with the USB standard, the amount of power that can be transferred via a USB connection is limited according to thresholds set forth in the USB specification and its revisions. In the first version of the USB standard, the amount of current that could be transferred via a USB connection was limited to 500 mA. This current limit for USB connection was later increased to 900 mA and then to 1.5 A. The current limit for USB charging ports has recently been increased to 5 A. In some cases, the maximum available current may be provided by a USB hub as soon as a peripheral device is connected to one of its USB ports. In most instances, however, enumeration of the peripheral device by the host device determines the appropriate current level that should be provided by the USB hub. The USB hardware of the host device must be capable of supporting these various current limit settings and providing a regulated supply of power that stays below the threshold current limit that is applicable for a USB port at any given time.
USB hubs implement power switching circuitry that is configured to enforce the applicable current limits on the USB ports that are supported by a hub. In general, power switching turns off the power supplied to a USB port when the current drawn by a peripheral device connected to the USB port exceeds a current limit. Two different mechanisms may be used by USB hubs to implement power switching.
Individual mode power switching implements a dedicated power switching mechanism for each USB port. Each USB port has an individual power switching mechanism that measures the current drawn by a peripheral device connected to the USB port and shuts off power provided by the port's USB connection if the measured current draw exceeds the current limit that is being enforced for this port.
The second power switching mechanism is gang mode power switching, where the USB hub regulates and controls power to a group of USB ports on a collective basis. In ganged mode power switching, the USB hub measures the total current that is collectively drawn by the peripheral devices connected to the ganged USB ports. If the measured, aggregate current drawn by the ganged USB ports exceeds the current limit being enforced, the USB hub shuts off power to all of the ganged USB ports.
With regards to the cost required to implement over current protection, gang mode power switching is preferable to individual mode switching since implementing a gang mode power switch requires only a single power switch and a single current sensing component that measures the current being drawn by all ports, while individual mode requires a separate power switch and current sensing component for every USB port. Despite the cost savings provided by gang mode power switching, as USB charging ports become more prevalent and the current limits being supported by USB ports increase, gang mode power switches place greater demands on the USB connectors used to deliver this power.
In particular, implementing gang mode power switching requires that the USB connectors that comprise each of the USB ports withstand currents up to the current limit being enforced by the single power switch and current sensing component. Since the current limit used by a gang mode power switch will typically be compared to the sum of the current draws on all of the supported USB ports, this current limit is typically set as high as possible in order to support simultaneous current draws on the supported USB ports. Moreover, the current limits that must be supported by USB power port switches have gradually increased as the current draws supported by USB have been increased. However, many USB connectors presently being sold were originally designed for use at the lower current levels in use prior to changes in the USB standard. As a result, USB connector components used in gang mode power switches may be exposed to current levels that exceed the current ratings for the USB connectors, thus stressing these components and increasing malfunctions and failures.
Individual power mode switching places less stress on the USB connectors since each USB port has a dedicated power switch and current sensing unit that is configured to protect an individual USB port. This allows the current limit settings in individual mode power switches to be set according to the current rating of the connector hardware of the individual USB port being protected and the properties of the peripheral device connected to the USB port, without concern for providing sufficient current for multiple USB ports to operate concurrently. Thus, there exists a need for USB power switching that can provide the costs benefits of gang mode power switching while providing the reduced stress on USB connectors and individualized protection that is provided by individual mode power switching.